1. Field of the Invention
The present invention relates to CRT display devices and, more particularly, to a CRT display device having a Hi-Gm tube which is capable of obtaining an ordinary intensity of current under a low drive voltage.
2. Description of the Background Art
FIG. 6 is a block diagram showing a constitution of a conventional CRT display device. In FIG. 6, reference characters 17, 2, 3, 4, 6, 7, 9, 10, 11, 13, 18 and 19 denote a CRT, a cathode, a G1 electrode, a G2 electrode, a G3 electrode, an anode, a video cathode amplifier, a cathode bias voltage source, a video input, an adjustment input, a flyback transformer and a resistor, respectively. An electron gun which irradiates an electron beam on a phosphor screen comprises the cathode 2, the G1 electrode 3, the G2 electrode 4 and the G3 electrode 6. The cathode 2 is provided with cathodes for red, green and blue each of which emits the beam for hitting the phosphor screen of red, green and blue.
Next, FIG. 6 is explained below. The video signal input 11 is inversely amplified by a video cathode amplifier 9 and then capacitor-coupled. The thus capacitor-coupled input is applied with the cathode bias voltage in accordance with the adjustment input 13 by the cathode bias voltage source 10 and then inputted to the cathode 2. On the other hand, the anode 7 is applied with a high voltage of about 25 kV which has been boosted by the flyback transformer 18. This high voltage of the anode 7 of the CRT 17 (hereinafter referred to as CRT anode high voltage) is created by boosting and then rectifying a horizontal retrace pulse generated by a horizontal deflection output circuit. The G2 electrode 4 is applied with a voltage of about 700 V to about 1000 V generated by dividing the voltage of about 25 kV, which has been boosted by the flyback transformer 18, by the resistor 19. In the conventional CRT display device, since it is characteristic that a current does not flow in the G2 electrode 4, the resistor 19 for dividing the high voltage is about 100 Mxcexa9.
Under a condition that the voltage to be applied to the cathode 2 (hereinafter also referred to as cathode voltage) is changed while respective voltages to be applied to the G1 electrode 3, the G2 electrode 4, the G3 electrode 6 and the anode 7 are held to be constant, when the cathode voltage becomes lower than a specified level, the electron beams emitted from the cathode 2 start flowing in a direction of a screen. The resultant flow of the electron beams from the cathode 2 in the direction of the screen is called as a beam current. The state in which the beam current is flowing shows that the beam hits the phosphor screen comprising phosphors of red, green and blue thereby allowing the screen to light. When the beam current flows in volume, the electron beam which reaches the phosphor screen is increased in number so that luminance of the screen is enhanced. In contrast, when the beam current is scarcely flowing, the luminance of the screen is decreased whereupon a video to be displayed on the screen turns to be dark. A display level of the image in which a dark screen starts lighting is called a black level. A voltage which is applied to the cathode 2 so as to display the black level is called as a black level bias voltage or a cutoff voltage.
In the conventional CRT display device, processing of adjusting the black level bias voltage called as a cutoff adjustment is performed by adjusting the cathode bias voltage to be applied to the cathode 2. A black level bias voltage value of the cathode has a variance ,for example, between 80 VDC and 110 VDC in each of the electron guns (cathodes) for R (red), G (green) and B (blue) depending on production process of the CRT. Unless such variance is corrected, a specified black color can not be displayed on the screen. The cutoff adjustment is an adjustment which allows a point in which the beam starts lighting and the black level of the video signal to agree with each other and also a processing operation which is performed for allowing the cutoff voltage of each of electron guns for R, G and B to agree with the black level of each signal so as to correctly represent a black portion and a dark portion of an image. Specifically, a coarse adjustment is first performed by adjusting a G2 electrode voltage such that a point in which the beam starts lighting to some extent is adjusted (or the G2 electrode voltage is fixed). The black level bias voltage value to be applied to each of cathodes for R, G and B is next adjusted thereby allowing the video on the screen of the CRT (hereinafter also referred to as CRT screen) and the luminance of the black color to agree with each other.
On the other hand, Japanese Patent Laid-Open No. 224618/1999 discloses a high-luminance CRT (hereinafter also referred to as Hi-Gm tube) in which a modulation electrode (hereinafter referred to as Gm electrode) is further provided between the G2 electrode and the G3 electrode. FIG. 7 is a block diagram showing a constitution of the Hi-Gm tube. In FIG. 7, reference characters 20, 21, 22, 23, 24 and 25 denotes a G1 electrode, a G2 electrode, a G3 electrode, a cathode, an electron emissive material provided on a surface of the cathode and the newly provided Gm electrode, respectively. Electrodes after the G3 electrode and the constitution as a whole are the same as those of a conventional electron gun.
FIG. 8 illustrates a potential distribution on a rotation symmetry axis in the proximity of the cathode of the Hi-Gm tube. In FIG. 8, the abscissa axis and the ordinate axis show a position (distance) (mm) from the cathode 23 and potential (V), respectively. Reference characters 26, 27 and 28 shown in FIG. 8 denote potential (electric field), a region in which the Gm electrode exists and an area in which the potential is low, respectively. Further, a dashed line shown in FIG. 8 shows potential of the cathode 23, that is, the cathode bias voltage. In the Hi-Gm tube, the Gm electrode 25 is disposed in the region shown by the reference character 27 which lies in about 0.5 mm inclusive of its vicinity far from the cathode 23. The potential 26 of the region 27 in which the Gm electrode 25 is disposed is determined by setting a direct-current voltage (DC potential) of the Gm electrode 25 at a specified voltage value, for example, 80 V. When the cathode voltage (dashed-lined portion) is changed while the Gm electrode voltage is fixed at 80 V, a quantity of the electrons which proceeds in the direction of the screen can be controlled. That is, when the potential of the cathode shown by the dashed line becomes smaller than the potential (electric field), the electrons flow whereas, when the potential of the cathode becomes larger, the electrons do not flow. It should be noted that the potential (electric field) is changed as the voltage to be applied to the Gm electrode 25 is changed.
As shown in FIG. 8, in a side of the Gm electrode 25 facing the cathode 23, electrons always exist in volume in an operating area of the cathode. Moreover, potential gradient after passing through the Gm electrode 25 is about one digit larger than that between the cathode and the G1 electrode of a conventional type. That is, the electrons which have passed in the proximity of the Gm electrode 25 do not suffer from an influence of a space charge effect whereupon many of them can proceed in the direction of the screen. Therefore, the current flowing in the direction of the screen depends on a quantity of electrons which can pass through a position where the Gm electrode 25 exists and whose potential is the lowest. By the reason described above, the same beam current as a conventional one is allowed to flow by half or less the conventional potential difference of the cathode 23. In other words, when the potential difference is the same as conventional, twice or more the conventional beam current is allowed to flow.
FIG. 9 shows a relation of each current vs cathode voltage of a Hi-Gm tube. In FIG. 9, reference characters 29, 30, 31 and 32 are a cathode current, a beam current, a G2 electrode current and a Gm electrode current, respectively. Voltage values of electrodes shown in FIG. 9 are set such that a G1 electrode voltage, a G2 electrode voltage, a Gm electrode voltage and a G3 electrode voltage are 0 V, 500 V, 80 V and 5.5 kV, respectively. As shown in FIG. 9, the more the cathode voltage is lowered, the more the beam current 30 flows thereby increasing the luminance of the screen. Further, the Gm electrode current 32 or the G2 electrode current 31 also flow in proportion to the beam current 30. Furthermore, it is also shown that, even when the beam current 30 does not flow at the time the cathode voltage is 80 V , the cathode current 29 flows into the G2 electrode. That is, it is understood that the difference between the cathode current 29 and the beam current 30 shown in FIG. 9 is equal to a sum of currents flowing in the G2 electrode and the Gm electrode.
In a case of the CRT display device using the Hi-Gm tube as described above, the Gm electrode is newly added to the conventional electron gun. In such display device using the Hi-Gm tube, it is necessary to additionally take a cutoff adjustment method using the Gm electrode potential into consideration.
Since the Hi-Gm tube allows the current twice or more as large as the conventional one to flow by the same cathode voltage amplitude as the conventional one and, moreover, sensitivity of the region in which the beam starts lighting is large, the luminance visually fluctuates to a great extent as the Gm voltage fluctuates. For example, when the potential of the Gm electrode is decreased, the point in which the beam starts lighting is decreased; that is, the black level on the screen is lowered whereupon the black color appears to be subsided. To contrast, when the potential of the Gm electrode is increased, the point in which the beam starts lighting is heighten whereupon the black color appears to be stood up like a noise.
As shown in FIG. 9, when electrons flow in the direction of the screen, that is, when the beam current flows, the current flows into the Gm electrode of the Hi-Gm tube and the beam current fluctuates in accordance with the luminance. On an occasion as described above, there exists a possibility that, when the current flowing into the Gm electrode fluctuates, a voltage source for applying a voltage to the Gm electrode allows an output voltage (Gm electrode voltage) thereof to fluctuate by being influenced by the resultant current fluctuation. There exists a problem that, when the Gm electrode voltage which the voltage source applies to the Gm electrode fluctuates, a level in which three beams start lighting relatively fluctuates whereupon a color temperature change and a luminance shift (change) may be brought about in a black color side. To solve the above-described problem, in the CRT display device using the Hi-Gm tube which is capable of allowing a large beam current to flow by a little potential difference, needed is the voltage source for keeping a supply voltage thereof to be applied to the Gm electrode to be constant, irrespective of the fluctuation of the current flowing into the Gm electrode caused by the luminance change.
Further, the current flows into the G2 electrode of the Hi-Gm tube in the same way as in the Gm electrode even when the electrons flow in the direction of the screen. Furthermore, the current value fluctuates in accordance with the luminance in the same way as in the Gm electrode. When the Hi-Gm tube is used, a current of 0.1 mA to 0.9 mA flows in the G2 electrode in a steady state whereupon, in a conventional method of dividing the voltage boosted by the flyback transformer by a resistor, a potential drop by the resistor becomes large. Moreover, an anode voltage and a focus voltage also fluctuate. When the G2 electrode voltage fluctuates, the beam current which is the current of electrons in the direction of the screen fluctuates and, accordingly, the luminance changes. Furthermore, when the G2 electrode voltage fluctuates, focus characteristics are affected to some extent. Therefore, needed is the voltage source which keeps the supply voltage thereof to be applied to the G2 electrode to be constant, irrespective of the fluctuation of the current flowing into the G2 electrode caused by the luminance change.
In the CRT display device using the Hi-Gm tube, twice or more the beam current is allowed to flow by providing a conventional cathode amplitude but, since the current flows also into the G2 electrode and the Gm electrode and, further, the current changes in accordance with the luminance, the cathode bias voltage source which is capable of allowing more than severalfold current to flow is needed. When an overcurrent which exceeds the capacity of the cathode bias voltage source flows thereinto via G2 electrode, there exists a problem that, not only output amplitude of the cathode bias voltage source is decreased, but also frequency characteristics are deteriorated thereby aggravating a video quality.
The present invention has been achieved in order to solve the above-described problems. A first object of the present invention is to provide a CRT display device which executes a cutoff adjustment in view of characteristics of a Hi-Gm tube. A second object of the present invention is to provide a CRT display device, comprising a voltage source that keeps a voltage to be applied to a Gm electrode to be constant, irrespective of a fluctuation of a current flowing into the electrode, which solves a problem that a color temperature change and a luminance change of a displayed video are brought about by a fluctuation of a Gm electrode voltage. A third object of the present invention is to provide a CRT display device, comprising a voltage source that keeps a voltage to be applied to a Gm electrode to be constant irrespective of a fluctuation of a current flowing into the electrode, which solves a problem that a luminance fluctuation is brought about by a fluctuation of a G2 electrode voltage. A fourth object of the present invention is to provide a CRT display device which solves a problem that, when an over-current flows into a cathode bias voltage source via G2 electrode, not only an output amplitude of the cathode bias voltage source is lowered, but also frequency characteristics are deteriorated thereby aggravating a video quality. Further, a fifth object of the present invention is to propose a cutoff adjustment method of a Hi-Gm tube.
A CRT display device according to the present invention comprises a CRT including an electron gun having a cathode for each of red, green and blue which emits a quantity of electrons in accordance with an applied voltage in a direction of a phosphor screen in which phosphors of red, green and blue are disposed, a G1 electrode, a G2 electrode and a G3 electrode each of which is provided in said direction of the phosphor screen from said cathode and forms an electric field by being applied with a specified voltage, and a modulation electrode which is disposed between the G2 electrode and the G3 electrode and which changes the electric field formed by each of the G1 electrode, the G2 electrode and the G3 electrode in accordance with an applied voltage, a modulation electrode voltage source which applies a voltage having a predetermined, specified voltage value to the modulation electrode, and a cathode voltage source which sets a voltage value such that it becomes the same as a modulation electrode voltage value to be applied to the modulation electrode by the modulation electrode voltage source and, further, applies a black level bias voltage value that is determined by a cutoff adjustment which finely adjusts the voltage value for the cathode for each of red, green and blue such that a black color of a displayed video when a video signal is in a black level agrees with a specified black color to the cathode when the video signal is in the black level.
A CRT display device according to the present invention comprises a CRT capable of allowing a great number of electrons to flow with a small cathode amplitude in a direction of a phosphor screen, including an electron gun having cathodes each of which emits a quantity of electrons in accordance with an applied voltage in the direction of the phosphor screen, a G1 electrode, a G2 electrode and a G3 electrode each of which is provided in the direction of the phosphor screen from the cathodes and forms an electric field by being applied with a specified voltage, and a modulation electrode which is disposed between the G2 electrode and the G3 electrode and which changes the electric field formed by each of the G1 electrode, the G2 electrode and the G3 electrode in accordance with an applied voltage, a modulation electrode voltage source which applies a voltage having a predetermined, specified voltage value to the modulation electrode, and a cathode bias voltage source which applies a voltage according to a video signal inputted from outside to the cathodes, irrespective of existence of electrons flowing into the G2 electrode and a Gm electrode among electrons which are emitted and proceed in the direction of the phosphor screen.
A CRT display device according to the present invention comprises a CRT capable of allowing a great number of electrons to flow in a direction of a phosphor screen with a small cathode amplitude, including an electron gun having cathodes each of which emits a quantity of electrons in accordance with an applied voltage in the direction of the phosphor screen, a G1 electrode, a G2 electrode, a G3 electrode and an anode electrode each of which is provided in the direction of the phosphor screen from the cathodes and forms an electric field by being applied with a specified voltage, and a modulation electrode which is disposed between the G2 electrode and the G3 electrode and changes the electric field formed by each of the G1 electrode, the G2 electrode and the G3 electrode in accordance with an applied voltage, an anode electrode voltage source for applying a specified voltage to the anode electrode, a G2 electrode voltage source which applies a voltage having a predetermined, specified voltage value to the G2 electrode, irrespective of existence of electrons flowing into the G2 electrode among electrons which are emitted from the cathodes and proceed in the direction of the phosphor screen, and a cathode bias voltage source which applies a voltage in accordance with a video signal inputted from outside to the cathodes, irrespective of existence of electrons flowing into the G2 electrode and a Gm electrode among electrons which are emitted from the cathodes and proceed in the direction of the phosphor screen.
A cutoff adjusting method according to the present invention is capable of being executed in a CRT display device including an electron gun having cathodes for red, green and blue each of which emits a quantity of electrons in accordance with an applied voltage in a direction of a phosphor screen provided with phosphors of red, green and blue, a G1 electrode, a G2 electrode and a G3 electrode each of which is provided in the direction of the phosphor screen from each of the cathodes and forms an electric field by being applied with a specified voltage, and a modulation electrode which is disposed between the G2 electrode and the G3 electrode and changes the electric field formed by each of the G1 electrode, the G2 electrode and the G3 electrode in accordance with the applied voltage, for allowing a black level of a video signal to be inputted to the cathodes for red, green and blue to agree with a black level bias voltage applied to the cathodes for red, green and blue such that electrons from the cathodes do not reach the phosphor screen, wherein the cutoff adjustment method comprises the steps of determining a modulation electrode voltage value to be applied to the modulation electrode such that it becomes a specified voltage value, setting a black level bias voltage value to be inputted to the cathodes for red, green and blue such that it becomes the same as the voltage value determined in the step of determining said modulation electrode voltage value, when the video signal of red, green and blue to be inputted to the cathodes for red, green and blue is at the black level, and finely adjusting the black level bias voltage value for each of the cathodes for red, green and blue such that a displayed image to be represented on a display screen of the CRT display device becomes a predetermined, specified black color, when the black level bias voltage is inputted to the cathodes for red, green and blue.